my milwaukee m18 fuel hammer drill just flashes red and green on the battery and won't run at all, what's wrong?

Diagnosis

The alternating red and green flashing lights on your M18 battery pack indicate a critical fault detected by the Battery Management System (BMS). This is not a tool failure in the 2904-20 hammer drill itself, but a safety lockout triggered by the battery's internal protection circuit. The BMS has detected that the voltage across the individual lithium-ion cells has drifted too far apart (voltage imbalance) or that the internal temperature has exceeded the safe operating threshold during a high-torque application.

This most commonly occurs when the 2904-20 is pushed to its absolute limit—such as driving large diameter hole saws or large lag bolts into dense hardwood—causing the cells to overheat rapidly. In other cases, it happens if a battery has been left discharged for several months, causing one or more cells to drop below the critical 2.5V threshold. Once the BMS trips this "fault state," it opens the circuit to prevent permanent cell damage or thermal runaway, rendering the drill completely unresponsive regardless of the trigger pull.

How to Fix It

To attempt a reset of the BMS and recover the battery pack, follow these precise steps:

1. Remove the battery pack immediately from the 2904-20 drill. Do not attempt to "force" the tool to run by repeatedly pulling the trigger, as this can further degrade a weak cell.
2. Place the battery on a flat, non-conductive surface at room temperature. Allow it to sit for at least 60 minutes. This ensures that any thermal trip is fully reset and the internal chemistry has stabilized.
3. Locate a genuine Milwaukee M18 Rapid Charger (such as the 48-11-1812). Ensure the charger is plugged into a stable power source and is not overheating.
4. Insert the battery into the charger. Observe the LED indicator. If the charger flashes red, the battery is severely under-voltage. If it begins a normal charging cycle (solid red or blinking green), let it charge to 100% without interruption.
5. Once fully charged, re-insert the battery into the 2904-20. Perform a "soft start" by pulling the trigger halfway before applying full load to ensure the connection is stable.

If That Didn't Work

• Clean the Contact Points: Use a cotton swab dipped in 90% isopropyl alcohol to clean the gold-plated terminals on both the battery and the base of the drill. Carbon buildup or sawdust debris can create high resistance, tricking the BMS into thinking there is a voltage drop, triggering the red/green flash.
• Cross-Test with a Known Good Battery: Insert a different M18 battery into the 2904-20. If the drill runs perfectly, the original battery has a failed cell and must be replaced. If the second battery also flashes red/green, the issue is likely a short in the drill's trigger assembly or the motor controller.
• Deep Cycle Attempt: If the charger refuses to recognize the battery, try "pulsing" it. Insert the battery, remove it for 5 seconds, and re-insert it. Sometimes this can jumpstart the charging circuit if the voltage is just barely below the charger's minimum detection threshold.
• Check for Physical Bloating: Inspect the plastic casing of the battery pack. If the seams are slightly separating or the plastic looks bowed, the cells have physically expanded due to overheating. This is a permanent hardware failure; the battery is unsafe and must be recycled.

Heads Up

• Avoid Deep Discharge: Never store your M18 batteries completely empty. If the voltage drops too low, the BMS will permanently lock the pack to prevent fire risks during recharging.
• Manage Heat: When using the 2904-20 for heavy hammer drilling in concrete, allow the tool to idle for 30 seconds between holes to let the internal fan clear out heat from the motor and battery interface.
• Charger Quality: Avoid using third-party "knock-off" chargers. These often lack the precise voltage regulation required by the M18 BMS and can trigger the red/green fault light by overcharging specific cells.
• Storage Temperature: Store your batteries in a cool, dry place. Extreme cold (below 32°F) or extreme heat (above 120°F) can cause the voltage imbalance that leads to this specific error code.